The New Covenant as an unbreakable covenant in Christ’s blood
One attractive reformed Baptist argument against paedobaptism sounds something like this: because the New Covenant is in Christ’s blood and because he does not fail in his role as mediator to save all of his covenant people, there are no unregenerate individuals within the New Covenant community. The covenant community is not mixed but is purely regenerate. This is an attractive and seemingly convincing argument for numerous reasons, especially to someone who holds to reformed soteriology. After all, our Lord himself asserted that he would not lose a single one of those whom the Father has given him. (John 6:39) The Scriptures clearly teach that the New Covenant is a better covenant and that Christ will not lose any that he has died for, those he intercedes for before the Father. Reformed theologians all agree that the elect cannot be lost and, in this sense, it is impossible to “break” the New Covenant because God himself fulfills all of the covenant conditions for us. Christ has kept the whole law for us and his perfect righteousness is imputed to us by faith which is a free gift of the Spirit.
However, where this argument fails is in a confusion between the administration of the covenant with its actual substance. The New Covenant must be administered, meaning that the New Covenant has a visible component where individuals become members of a visible gathering of the Lord’s people and receive the visible sign of the covenant. The word “administration” may be an “extra-biblical” word just as the word “Trinity” or even the idea of God’s “personhood”, but the idea of the administration of the covenant is seen clearly in statements like the Apostle John’s that some “went out from us, but they were not of us.” (1 John 2:19 KJV) The same language is used of by Paul to make the same point in Romans 9:6 when he asserts that “they are not all Israel who are descended from Israel.” (NASB) On the other hand, the substance of the covenant are those who have been predestined by the Father, purchased by the Son, and regenerated by the Spirit. The below diagram is helpful in understand this membership in the covenant.
So can the New Covenant be broken? Clearly, the administration can be. Visible members of the covenant community can spurn their profession and use of the covenant signs, heaping more judgment on themselves. This is why the New Testament uses examples of Old Testament faithlessness that resulted in condemnation in order to warn the New Testament professor of falling away. (1 Cor. 10:5-7; Heb. 10:28-31) Indeed, in an effort to encourage the new covenant member to persevere in the faith, the writer of Hebrews asserts “The Lord will judge his people.” (10:30) This can only be understood if one grasps the distinction between God’s covenant people visibly (administration) and his covenant people invisibly (substance).
Why does this have a voice in the baptism discussion? The idea that the Lord only covenants with the elect in the New Covenant is used to argue that the sign should only be given to those who show evidence of being regenerate. The argument goes that the New Covenant is different than the Old in that it cannot be broken and is only made with the elect and therefore does not include the children of believers like the Old. But as shown above, the weight of this argument does not hold and is therefore not a credible argument to exclude the children of believers from membership within the covenant community and therefore the covenant’s initiating ordinance.